This invention relates to guidewires for use with catheters in blood vessels or other body lumens. For example, such guidewires and catheters commonly are involved in various cardiovascular procedures. More particularly, the invention concerns a guidewire which can be steered into and along body passageways such as a narrow blood vessel (including naturally narrow vessels as well as stenosed vessels) to locate the distal end of the guidewire in a precise position at a target site. Once so placed, a catheter can be advanced over the guidewire directly to the target site. The invention is of particular importance for use in coronary dilatation techniques where the catheter itself is very small in diameter and is difficult to advance and place deeply in the patient's coronary arteries.
A wide variety of guidewires are well known in the prior art. An example of a tapered guidewire having a relatively flexible tip portion and a somewhat more rigid body portion is disclosed in U.S. Pat. No. 4,345,602 to Yoshimura et al. An example of a guidewire for use in coronary angioplasty is described in U.S. Pat. No. 4,545,390 to Leary the teachings of which are incorporated herein, in its entirety, by reference. The Leary patent discloses a small diameter, steerable guidewire, the major portion of which is a flexible, substantially torsionally rigid shaft having a tapered distal portion. The tapered portion is surrounded by a helically wound coil which is attached to the shaft at its proximal and distal ends respectively. Additionally, a portion of the coil extends beyond the distal end of the tapered portion and serves as a highly flexible segment to avoid trauma or damage to a blood vessel through which the guidewire is advanced. The distal tip of the guidewire can be bent to a predetermined shape prior to use by a physician to enhance the steerability of the guidewire.
As described in European Patent Application No. 141,006 to Terumo, conventional prior art guidewires used with catheters commonly include flexible coils formed of wire. In a typical procedure, such a guidewire is inserted percutaneously into a blood vessel, typically using a needle, then the guidewire is manipulated and advanced to a target site. A catheter then is introduced into the blood vessel along and following the path of the guidewire to the target site. Among the difficulties sometimes encountered with conventional guidewires, is the possibility that the distal end of the guidewire may kink as it is advanced through the patient's vasculature. Kinking is the result of a plastic deformation of the guidewire and usually is characterized by a sharp deformation or point bend of the very distal section of the wire. Such a deformation may result from attempting to pass a guidewire through a relatively hard, calcified lesion, a mostly occluded vessel section or a very tortuous vascular section. The wire may kink or bend back upon itself in a condition referred to as prolapse. Thereafter, the wire may return to its original shape, or it may remain permanently deformed if, during the bending, the wire material is bent beyond its elastic limit.
Once kinking occurs, the guidewire is usually discarded because it cannot be adequately straightened for use. Typically that is because of the plastic deformation of the wire or because the physician does not want to spend the time necessary to attempt to straighten the kinked guidewire. Consequently, the procedure may have to be aborted and a new guidewire selected, reinserted, and again manipulated and advanced to the target site. Reinsertion of another guidewire increases the risk of trauma to the blood vessels. Unless great care is taken, the blood vessels can be seriously damaged.
It is important the guidewire be sufficiently flexible so that it does not damage the wall of the blood vessel and so that it can adapt itself to the path of the blood vessel into which it is being inserted.
Additionally, in many instances, it is desirable to provide a guidewire having a curvature or some other shape at its distal end to assist the physician in introducing, advancing and steering the guidewire and catheter to the target site in the blood vessel. However, because of the curvature of the wire, and the resistance of the wire to being straightened when the guidewire is drawn into the catheter and thereafter introduced into the blood vessel, undesirably high friction can occur between the guidewire and catheter. This increases the likelihood of kinking of the guidewire.
It has been suggested that the foregoing difficulties can be addressed by forming guidewires of superelastic alloys. An example of one such guidewire is found in the above-mentioned Terumo European patent application.
Superelastic alloys display a property referred to as stress-induced martensite (SIM). When alloys exhibiting SIM are stressed at a temperature above M.sub.s, a temperature denoting the start of martensite transformation, but below the maximum temperature at which martensite formation can occur under stress (denoted M.sub.d), they will first deform elastically and then, at a critical stress, begin to transform by the formation of stress-induced martensite. Depending upon whether the temperature is above or below A.sub.s, the austenite transformation temperature, the behavior of the material will differ when the deforming stress is released. If the temperature is below A.sub.s, the stress-induced martensite is stable; but if the temperature is above A.sub.s, the martensite is unstable and will transform back to austenite, with the sample returning or attempting to return to its original shape. This effect is seen in almost all alloys which exhibit a thermoelastic martensitic transformation. However, the extent of the temperature range over which SIM is seen, and the stress and strain ranges for the effect vary greatly with the particular alloy selected. Alloys displaying SIM frequently are referred to as pseudoelastic or superelastic alloys.
One problem with guidewires made of an SIM alloy is that, unlike the conventional prior guidewires, guidewires formed of such alloys cannot be readily formed immediately prior to surgery into a shape desired for a specific procedure. This is because the SIM property which is so desirable in prevention of kinking serves to preclude formability by the physician. Accordingly, there is a need for a guidewire that combines the advantages of guidewires formed of superelastic alloys with the ability to form or shape the distal end of the guidewire immediately prior to use by the physician.